The invention relates to a process and to a device for working a workpiece wherein an abrasive liquid is sprayed onto the workpiece, via at least two nozzles, each of which is disposed at an angle with respect to the workpiece and the liquid jets from which intersect one another on or below the workpiece surface, such as for example for shaping or polishing optical components.
It is known to form curved optical surfaces in optical materials, such as quartz or glass, by means of grinding and polishing. In a three-component process of this nature, a tool, such as a mould, is used to press abrasive particles in a slurry onto the optical surface which is to be worked. The tool is subjected to load and is moved with respect to the workpiece. Although the known process makes it possible to accurately work the optical components, it is relatively lengthy. Furthermore, more complex shapes, such as aspherical optical components, cannot easily be formed using the known method.
A process and device according the preamble of claim 1 is known from DE-A-4407271. In this patent application a process is described for the working of surfaces at pressures ranging from 600-4000 bar. The high pressure used in the known process results in a relatively large roughness of the worked surface, such that the known process will not be suitable for shaping or production of for instance an optical component.
Therefore, one object of the present invention is to provide a process and device with which a workpiece can be shaped, ground or polished accurately and quickly. A further object of the present invention is to provide a process and device of this nature with which it is easy to impart complex shapes to a workpiece, in particular to optical components made of a refractive optical material, such as quartz, glass or plastic, or of a reflective optical material, such as metals and ceramic materials. Yet another object of the present invention is to provide a process and device which allow the surface to be shaped in a single operation and to be polished with the desired level of accuracy, for example to a roughness of 1 nanometre RMS or better.
To this end, the process according to the invention is characterized in that the abrasive liquid is sprayed onto the workpiece at a pressure of less than 50 bar, preferably of less than 20 bar, to shape and/or polish the surface of the workpiece. xe2x80x9cAbrasive liquidxe2x80x9d is in this context intended to mean a liquid which can be used to grind a surface to a relatively high roughness or to polish it to a lower roughness.
Surprisingly, it has been found that the abrasive liquid provides very controlled working of the surface of the workpiece at relatively low pressures, such as 50 bar or lower. The abrasive liquid, which preferably contains abrasive particles, has a low velocity at these low pressures, so that material is removed in a controlled manner without forming irregular pitting in the surface.
At the point where the liquid jets cross or intersect one another, the impulse of the abrasive particles or polishing particles is reduced to such an extent that no further material is removed below this point. In this way it is possible to set the working depth very accurately.
It has been found that the process according to the present invention makes it possible, when the abrasive liquid used is water containing silicon carbide particles with a size of approx. 20 xcexcm as the abrasive, to polish a surface of BK7 to an ultimate roughness of 1.5 nm RMS. A conventional polishing method with a particle size of this nature results in a roughness of approx. 5 xcexcm.
It should be noted that a device for cutting glass using a high-speed jet of liquid is known per se from American patent U.S. Pat. No. 4,787,178. However, the nozzle pressures which are used for cutting the glass are in the order of magnitude of 2000 bar. For this reason, the method is unsuitable for very accurate surface-working operations.
It is also known from American patent U.S. Pat. No. 5,573,446 to shape optical components by moving a stream of gas which contains abrasive particles over the surface of the workpiece in a raster pattern. This shaping process only provides limited accuracy, so that an optical component has to be polished separately after it has been shaped.
The process according to the present invention differs from the above methods by the fact that material is removed in a very controlled manner, making it possible, within a short time, both to shape the workpiece and to polish it until the desired roughness is reached.
The abrasive liquid according to the present invention may comprise a number of liquids, such as water or an organic liquid, such as octanol. Preferably, abrasive particles or polishing particles are added to an abrasive liquid, such as for example #800 silicon carbide or particles which have similar properties. Other suitable abrasive particles comprise diamond or aluminiun oxide, while diamond or cerium oxide can be used for polishing. The rate at which material is removed from the surface of the workpiece depends on the concentration, dimensions and hardness of the abrasive particles and on the type of abrasive liquid, the velocity of the abrasive liquid when it leaves the nozzle, the contact time, the geometry, the relative dimensions and orientation of the nozzle with respect to the workpiece surface, and the like. The abrasive-liquid pressures employed are preferably less than 50 bar, such as for example 5 bar. The diameter of the nozzle is preferably small compared to the dimensions of the workpiece, such as between 10 cm and 0.1 mm, preferably between 1 cm and 0.5 mm, and particularly preferably between 5 mm and 0.5 mm. The diameter of the workpiece may, for example, amount to 100 mm.
The operation is relatively insensitive to the distance between nozzle and workpiece.
Although the process according to the invention can be used on a multiplicity of materials, the method is particularly suitable for refractive optical materials, such as for example silicon, glass, sapphire, quartz, optical plastics, but also for reflective optical materials, such as metal or ceramic materials. Owing to the low energy of the abrasive liquid and the abrasive particles, material is removed gradually without pitting or scratches being formed. During the operation, one nozzle may be moved with respect to the workpiece, for example in a raster pattern. It is also possible to employ a series of nozzles and to rotate the workpiece about its axis of rotation at the same time. By linking the movement of the nozzle to the movement of the workpiece, it is possible to grind and polish complex geometric shapes, such as for example toric surfaces. By moving the axis of rotation of the workpiece it is possible, for example, to shape and polish a toric surface. The cross section of the nozzle may be circular, elliptical, triangular or rectangular, or may be in the form of a series of ellipses or rectangles in order to form a plurality of slots in a single production run, for example in order to form binary optical elements.